Cardiac output (CO) and Stroke volume (SV) are indicators not only for diagnosis of disease, but also for “real-time” monitoring of patients. Few hospitals are, therefore, without some form of equipment to monitor one or more of these cardiac parameters. Both invasive and non-invasive techniques are available.
Most of the techniques used to measure SV can usually be readily adapted to provide an estimate of CO as well, as CO is generally defined as SV times the heart rate HR. Conversely, most devices that estimate CO also estimate SV as a sub-step. As is explained in greater detail below, still another cardiac parameter that promises to provide clinically important information is stroke volume variation SVV. One way to estimate SVV is simply to collect multiple SV values and calculate the differences from measurement interval to measurement interval.
One common way to measure SV or CO is to mount some flow-measuring device on a catheter, and then to thread the catheter into the subject and to maneuver it so that the device is in or near the subject's heart. Some such devices inject either a bolus of material or energy (usually heat) at an upstream position, such as in the right atrium, and determine flow based on the characteristics of the injected material or energy at a downstream position, such as in the pulmonary artery.
Still other invasive devices are based on the known Fick technique, according to which CO is calculated as a function of oxygenation of arterial and mixed venous blood.
Invasive techniques have obvious disadvantages. For example, catheterization of the heart is potentially dangerous, especially considering that the patients typically have a serious condition. Moreover, some catheterization techniques, most notably thermodilution, rely on assumptions, such as uniform dispersion of the injected heat, that affect the accuracy of the measurements depending on how well they are fulfilled. Moreover, the very introduction of an instrument into the blood flow may affect the value (for example, flow rate) that the instrument measures.
Doppler techniques, using invasive as well as non-invasive transducers, are also used to measure flow and to calculate SV and CO from the flow measurements. Not only are these systems typically expensive, but their accuracy depends on precise knowledge of the diameter and general geometry of the flow channel Such precise knowledge is, however, seldom possible, especially under conditions where real-time monitoring is desired.
One blood characteristic that has proven particularly promising for accurately determining parameters, such as CO, SV, and SVV with minimal or no invasion is blood pressure. Most known blood-pressure-based systems rely on the so-called pulse contour method (PCM), which calculates an estimate of the cardiac parameter(s) of interest from characteristics of the beat-to-beat pressure waveform. In the PCM, “Windkessel” (German for “air chamber”) parameters (characteristic impedance of the aorta, compliance, and total peripheral resistance) are typically used to construct a linear or non-linear, hemodynamic model of the aorta. In essence, blood flow is analogized to a flow of electrical current in a circuit in which an impedance is in series with a parallel-connected resistance and capacitance (compliance). The three required parameters of the model are usually determined either empirically, through a complex calibration process, or from compiled “anthropometric” data, i.e., data about the age, sex, height, weight, and/or other parameters of other patients or test subjects. U.S. Pat. No. 5,400,793 (Wesseling, 28 Mar. 1995) and U.S. Pat. No. 5,535,753 (Petrucelli, et al., 16 Jul. 1996) discloses systems that rely on a Windkessel circuit model to determine CO.
PCM-based systems can monitor SV-derived cardiac parameters using blood pressure measurements taken using a variety of measurement apparatus, such as a finger cuff, and can do so more or less continuously. This ease of use comes at the potential cost of accuracy, however, as the PCM can be no more accurate than the rather simple, three-parameter model from which it was derived. A model of a much higher order would be needed to faithfully account for other phenomena. Many improvements, with varying degrees of complexity, have been proposed for improving the accuracy of the basic PCM model.
Vasoactive agents (such as vasoconstrictors, vasodilators, and inotropes) have an impact on vascular tone (vascular compliance and resistance), which usually induces changes in blood pressure. As a result, this could have a negative impact on blood-pressure-based systems that measure CO and introduces errors on the measurement parameters, such as CO, SV, SVR and SVV. Vasoactive agents are a group of bioactive chemicals, which change vasomotor tone through their influence on various peripheral receptors. Most of these agents have inotropic effects (e.g. norepinephrine) as they bind with receptors positioned on the surface of the myocardium. Vasoactive drugs generally affect stroke volume and heart rate, and, thus, determine cardiac output and overall cardiovascular function. When vasoactive drugs are present, CO, SV, and SVV measurements are often inaccurate.